An image forming apparatus employing a photosensitive sheet as a photosensitive-pressuresensitive type medium forms a latent image on a portion of a photosensitive sheet successively drawn from a photosensitive sheet roll, and by depressing the latent image bearing portion onto an image receiving sheet, forms a visible image on the image receiving sheet. Therefore, the portion of the photosensitive sheet after having been depressed onto the image receiving sheet is successively taken up for withdrawal since micro-capsules, which contain a dye intermediate and photosensitive substances therein and are coated on the photosensitive sheet, have been already used. Accordingly, in the above image forming apparatus, it is always necessary to recognize the remaining amount of the photosensitive sheet and display it so that the exchange of the photosensitive sheet roll may be made promptly when the sheet has been used up. Conventionally, those remaining amount detection devices have been developed as a device for detecting the amount of unused photosensitive sheet remaining in the photosensitive sheet roll, which are published in the following Japanese official gazettes:
For example, in a remaining amount detection device cited in Publication for Unexamined Utility Model Application No. 54-112986, a mark composed of magnetic substance or the like is predeterminately applied to the rear edge of a sheet which is wound up into a roll and for example, a magnetic detector is employed to detect and indicate the rear edge.
Moreover, in a remaining amount detection device cited in Application No. 55-143246, the remaining amount is shown by measuring the weight of the sheet roll by utilizing the fact that the sheet roll weight decreases accompanying the drawing of the sheet.
Furthermore, in a remaining amount detection device cited in Application No. 57-115036, by utilizing the fact that the diameter of a sheet roll gets smaller accompanying the drawing of the sheet, an actuator is depressed onto the circumference surface of the sheet roll, and thereby the remaining amount of the sheet is indicated according to the displacement of the actuator.
However, the remaining amount detection device cited in the above Application No. 54-112986 has a problem in that since the rear edge of the sheet is not detected until the sheet roll has been completely used up, the user do not notice the shortage of the sheet beforehand.
In the remaining amount detection device cited in the above Application No. 55-143246, it is difficult for the device to measure the accurate weight while supporting the sheet roll in a simple structure and permitting it to rotate freely. Besides, a smooth drawing of the sheet might be disturbed by supporting members for the sheet roll, and therefore the surface of the sheet might have scratches or specks. Especially in the case of photosensitive sheet as a photosensitive-pressuresensitive type medium, it is apprehended that micro-capsules containing photosensitive substances or the like therein might be broken. Consequently, it has a problem in that those scratches or specks might cause bad influence on the quality of images.
Moreover, in the remaining amount detection device cited in the above Application No. 57-115036, since the actuator is always depressed onto the surface of the photosensitive sheet at the outermost circumference of its roll, the depression might cause scratches or specks on the photosensitive sheet or the like in the same way as is described above.